Weekly Crop Update

University of Delaware Cooperative Extension

Volume 5, Issue 17

July 24, 1997

Field Crops:

Field Crop Insects -

Joanne Whalen, Extension IPM Specialist.




Last weeks hot weather resulted in an explosion of potato leafhopper adults and nymphs. In some fields, populations were extremely high and hopper burn has turned entire fields yellow. Once significant yellowing occurs, the best option for control is to cut and recheck the field for leafhoppers within one week of cutting. Although cutting can be used as a control strategy, it often does not reduce populations below economic levels when populations are extremely high. Since leafhoppers can cause significant damage to short alfalfa, an insecticide treatment should be applied if you find 20 leafhoppers per 100 sweeps in alfalfa 3-inches or less in height. The threshold in 4-6 inch tall alfalfa is 50 per 100 sweeps, 100 per 100 sweeps in 7-11 inch alfalfa, and 150 per 100 sweeps in alfalfa 12-inches or greater in height. Remember, a sweep net is needed to accurately sample fields ( hat samples usually get you in trouble ). If you need a net, please contact me and we can sell you one for $20. Ambush, Baythroid, dimethoate or Pounce will provide good control.



Economic levels of spider mites and/ or potato leafhoppers can be found in fields throughout the state. In many fields, a combination of mites, leafhoppers and thrips can be found below the threshold level for each individual pest. Although we generally do not treat for leafhoppers and/or thrips, a combination of these three pests, especially under drought stressed situations, can cause significant economic loss. As a general rule, the threshold for each pest should be reduced by 1/3 when more than one is present and fields are drought stressed. The following action thresholds should be used as a guideline when making a treatment decision: Spider Mites: 20-30 per leaflet and early signs of stippling damage; Potato Leafhopper: 8 per sweep (unstressed plants), 4 per sweep(stressed plants), and Thrips : 8 per leaflet. Recent weather conditions including moisture, high humidity and moderate temperatures (mid - high 70s) can reduce populations below the economic threshold and in some cases can even crash the population. However, populations can quickly resurge if the hot, dry weather returns next week. The bottom line is that fields should be scouted carefully next week . You will need to count the insect populations twice a week and treat as soon as you reach an economic threshold level. Since most plants should be in better condition, (i.e less drought stressed) it will be an ideal time for insecticides to work and the pest populations will be more susceptible to control. If leafhoppers and/or thrips are the predominant species, a pyrethroid will provide good control. However, if mites are also present dimethoate is still the material of choice. The addition of a crop oil or organosilicone will improve control, especially when applied by air. In areas where you still plan to plant double crop beans and/or if beans are still small, be sure to watch for grasshopper activity. Grasshoppers can quickly defoliate beans in these situations. The most effective choices from past years have been Asana, Sevin or Warrior.


Yield Losses from Drought Stressed Corn -

Richard W. Taylor, Extension Agronomist


Bob Uniatowski, Extension Associate - Field Crops



Many growers experienced an early season drought this year. Typical estimates of expected yield loss are from 1 to 2 percent per day of stress. However in a study conducted at Penn State, the effect of no rain from emergence to the 9th leaf stage (corn will be about 24 to 30 inches tall if water is available) was a 20 to 30 percent yield reduction from the hybrid's yield potential with no water stress (190 bu/A in this study). The study did not report the actual number of days that stress was visible on the plants.

 For other growth stages, the expected yield reduction from a hybrid's realistic yield potential will be as follows:

Sidedress Time: One day of water stress can result in a 2 to 4 percent yield reduction.

Pollination: One day of water stress can result in a 3 to 8 percent yield reduction. 

Blister Stage: One day of water stress can result in a 3 to 7 percent yield reduction.

Milk Stage: One day of water stress can result in a 3 to 5.5 percent yield reduction. 

Dough Stage: One day of water stress can result in a 3 to 5 percent yield reduction.


Corn Pollination -

Richard W. Taylor, Extension Agronomist


Bob Uniatowski, Extension Associate - Field Crops


A typical corn tassel produces 2 to 5 million pollen grains or up to 300 pounds of pollen per acre. During pollen shed, each individual silk (one grows from each ovule or future kernel on the ear) must emerge from the enclosing husk to capture a pollen grain. The silk is sticky and pollen readily attaches to each silk. Only one pollen grain is needed to pollinate each ovule (future kernel). A typical, well developed ear has 650 to 1,000 potential kernels arranged on an even number of rows around the cob. The pollen grain germinates and a tube grows down through the silk to the ovule where fertilization occurs, all within a 24 hour period. The ovule is now a kernel.

Silks grow 1 to 1.5 inches per day if adequate moisture is available. It takes about 2 to 3 days for all the silks on an ear to be exposed for pollination. Silks at the butt of the ear emerge first and silks from the ear tip emerge last.

Silks grow fastest from 9 pm to 6 am when water stress is lowest. Even with adequate available soil water, water stress increases as sunlight, temperatures, and air movement increase water need during the day. During this daily water stress, silk growth rate declines after 6 am and reaches a minimum rate by 3 pm. If available soil water is low due to drought conditions, corn will be slow to recover in the evening and silk growth rate can be severely reduced.

Pollen shed can last for 6 to 8 days and is very dependent on weather conditions. It occurs primarily during late mornings and early evenings when the tassel is dry. Pollen shed stops when the tassel is too wet or dry. Pollen is not washed off silks during rainstorms but more pollen is not shed until conditions again become favorable. Pollen shed begins 1 to 3 days before silk emergence but usually overlaps silk emergence by 5 to 7 days. About 95 percent of the pollen falling on a given ear comes from another corn plant.

Vegetable Crops:

Vegetable Crop Insects -

Joanne Whalen, Extension IPM Specialist.



Lima Beans.

Fields in the pin pod stage should be scouted twice a week for earworms, lygus bugs and stink bugs. Corn earworm catches are still relatively low; however, you should begin seeing egg laying in the next week to 10 days as moth activity increases. A drop cloth should be used to sample 6 foot of row in 5-10 locations throughout a field for corn earworm. For lygus and stink bugs, take 10 sweeps in 5-10 locations throughout a field. The treatment threshold for earworms is 1 per 6 foot of row. The treatment threshold for lygus and/or stink bugs is 15 adults and/or nymphs per 50 sweeps. Lannate is the material of choice for the control of all three insects. The rate of Lannate will depend on the average size of corn earworm larvae at the time of treatment. In most cases 1.5 - 2 pt of Lannate LV is adequate. However, if more than 1/3 of the larvae are large (> inch) then 3 pints per acre will be needed.



Corn borer sprays are needed on a 7-10 day schedule in areas where blacklight trap catches are above 3 per night. Pepper maggot controls will also be needed through the end of the month. At this time, Orthene will provide good corn borer , fall armyworm and pepper maggot control as well as help suppress aphids. If you have been using Ambush, Asana or Pounce on a routine basis, they will not provide effective fall armyworm or pepper maggot control. In addition, aphid populations are starting to increase and explosions can occur under hot, dry weather with constant pyrethroid use. If populations explode, Provado or Lannate will provide good aphid control. As corn earworm pressure increases, Orthene will not provide adequate corn earworm control so Lannate will become the best option.


Pickling Cucumbers.

We have just detected our first small pickleworm larvae in plant terminals. Young larvae are most easily detected in plants terminals, especially in weedy areas of the field or field edges. Timing a spray should be based on when larvae are first detected, the population level and the temperature. All pickleworm stages experience mortality at temperatures above 85 degrees. If worms are detected a few days from harvest and larvae are small, it is unlikely that worms will make it to the fruit. Young larvae feed in the growing tips or within blossoms, moving to the fruit when they are half grown. If more than one spray is needed, the first spray should be Lannate followed by Asana.


Processing Snap Beans.

We are starting to see an increase in corn borer moth flights in some areas and we often see an increase after a storm passes through the area. If trap catches continue to increase, all processing snap beans in the bud and pin stages should be treated with Orthene. Remember, Orthene will not provide effective corn earworm control. Once corn earworm BLT catches reach 20 per night or moths can be observed laying eggs in fields, Asana should be added to Orthene at the pin spray. After the pin spray, beans should be sprayed on the following schedule depending on local BLT information:



ECB Spray

Catches Interval

2-5 7 Day

6-10 6 Day

11-15 5 Day

    1. 3 Day


Sweet Corn.

Fresh market sweet corn should be sprayed on a 3-4 day schedule in Kent and Sussex Counties and on a 5 day schedule in New Castle county. In fields where Ambush or Pounce have been used, be sure to watch for increases in aphid populations. Warrior and Asana have provided good aphid control in sweet corn. If the hot, dry weather returns and aphid populations continue to increase, growers using Ambush or Pounce should alternate or use a combination with Lannate for one or two sprays.


U of D Crop Pest Hotline:

In State: 1-800-345-7544

Out-of-State: 302-831-8851


Vegetable Crop Diseases - Kate Everts, Extension Pathologist for Universities of Maryland and Delaware




The EPA has granted a specific exemption (Section 18) for the use of Acrobat MZ, Curzate M-8 and Manex C-8 to control late blight on tomatoes in Maryland. A five day pre-harvest interval must be observed for all of these fungicides. A maximum of 1.5 lbs of either Curzate M-8 or Manex C-8 may be applied. No more than 2.25 lbs of Acrobat may be used. Our weather has not been favorable for the development of late blight, however the rain we are having may change the situation. Continue to scout fields for late blight, and apply chlorothalonil or mancozeb as a protectant fungicide.



Weather prior to Wednesday has not been conducive to foliar disease development in vine crops. However, our rainy spell will make it necessary to tighten the spray schedule to every seven days. Our test of the weather based fungicide application model for gummy stem blight and anthracnose in Watermelon has saved two sprays at both Salisbury, MD and Laurel, DE. We are unable to see any differences in disease development at this time. We will continue monitoring disease, and take yield measurements. Plots will be available for viewing at a Twilight meeting on August 5 (5:30 p.m.). Extension Specialists and Agents will be on hand to discuss the fungicide model trial, the seeded observational watermelon trial, and the seedless watermelon variety trial, as well as answer questions about problems you have experienced this year. Dinner will also be provided. Please mark the date on your calender. For more details, call Charlotte Headley at 302/856-7303.


Just a Reminder...

Please remember to read pesticide labels carefully and abide by all restrictions (re-entry intervals for workers, and pre-harvest intervals). Also do not use products on crops for which they are not yet labeled. There are many reasons for this, perhaps the most important is that little local data is available on unlabeled products. Often we do not know how they will perform under Delmarva production and disease conditions. There may also be problems with crop injury, or companies may be registering a material for a specific use pattern. That information will only be available when the label is approved.

There are also many legal reasons for avoiding unlabeled material. If residue is detected on a truckload, an entire farm may be unable to sell produce. A public pesticide scare may decrease demand for a commodity and cause prices to plummet. The bottom line is that the label is a legal document, and should be strictly followed.


Crop Diagnostic Field Day

July 31, 1997

7:30 a.m. - 1:00 p.m.

University of Delaware Research & Education Center, Georgetown, Delaware.


Crop Management Tour

Thursday, August 21, 1997

9:00 a.m. - 12:30 p.m.

Free barbecue lunch to all participants.

Contact Charlotte at 302-856-7303




Late Blight Report.


DSV accumulation as of July 21, 1997 are as follows:




Location/ Emergence date



DSV=s July 17



DSV=s July 21





Baldwin - 4/20






10-day, high rate


Jackewicz - 4/26






10-day, high rate


Zimmerman - 4/28






10-day, high-rate


Baker - 5/7 -5/9






10-day, high-rate


Weather conditions are very unfavorable for late blight development. Late planted fields, of fields with early blight susceptible varieties would be the fields that may benefit from fungicide applications at the present time. Let's hope harvesting goes well and prices are high. Late blight information hotline number 1-888-831-SPUD.


Weather Summary

University of Delaware,


 Week of July 18 to July 24, 1997



0.50 inches: July 19

0.27 inches: July 23

0.22 inches: July 24

Readings taken for the previous 24 hours at 8 a.m.




Ranged from 95 F on July 19 to 73 F on July 24.


Ranged from 72 F on July 18 to 64 F on July 20 & 21.


Soil Temperature:

73.4 F. Average for the week.




 Complied & Edited By:

Tracy Wootten

Extension Associate - Vegetable Crops


Cooperative Extension Education in Agriculture and Home Economics, University of Delaware, Delaware State College and the United States Department of Agriculture cooperating, John C. Nye, Dean and Director. Distributed in furtherance of the Acts of Congress of May 8 and June 30, 1914. It is the policy of the Delaware Cooperative Extension System that no person shall be subjected to discrimination on the grounds of race, color, sex, disability, age or national origin.